Using the correct materials, methods, and tools makes a big difference in the final result. The photos on this page are from a crawl space in Atlanta that we encapsulated in 2008.CAUTION: Encapsulating a crawl space can create new problems if it's not done correctly. In some cases, a crawl space should not be encapsulated. Two big issues that must be looked at carefully are bulk water problems and the possibility of backdrafting combustion appliances in the crawl space. Pest control also places some restrictions on the details of encapsulation. For the best info available on this topic, go to Advanced Energy's website and download their guidelines, titled "Closed Crawl Spaces: An Introduction for the Southeast." Crawl space encapsulation is not suitable as a DIY project in most cases, so learn what you can and then call in a professional.
Step 1. The photo at left shows the crawl space before we encapsulated it. This ground had about 75% coverage with a 6 mil polyethylene vapor barrier and was also connected to a mechanical room and finished basement. The homeowners had tried numerous techniques and products to reduce the smells and humidity in the basement, mostly to little effect. They also had very cold floors in winter since there was no insulation in the floor. The only bulk water problem here was the result of a lazy HVAC contractor who ran the AC condensate line out of the mechanical room and into a corner of the crawl space. We installed a condensate pump and ran the drain all the way outside, which is what the HVAC contractor should have done to begin with. No combustion equipment meant no possibility of backdrafting in the crawl space.
Step 2. Here you see that we've put a drainage mat on the ground, mainly to protect the plastic that goes on top of it. We've also covered the walls with a transparent plastic, starting about 3 inches from the top and going down to the ground. This plastic is a multi-layer polyethylene with reinforcing scrim and will last much longer than regular plastic (6 mil poly). The plastic is fastened mechanically at the top, and we use an adhesive to seal the top edge to the foundation wall. (The one piece of foam board you see was installed sooner than usual because we were trying a new tool to attach it to the wall.) Although you can't see it, we've also sprayed a borate solution on the band joist and anywhere else that wood meets concrete. This protects the wood from termites, powder post beetles, carpenter ants, and wood decaying fungus.
Step 3. Now the floor plastic is going down over the drainage mat. The floor plastic also is made of multiple layers of polyethylene bonded together with a layer of reinforcing scrim embedded in the sheet. It's thicker than the standard 6 mil poly sheets in most crawl spaces and is virtually impossible to tear. It can get holes poked in it by sharp objects (easily repaired with tape), but it will last a long, long time in a crawl space. Where separate pieces of plastic meet, we overlap them by at least a foot and then seal the seam with a waterproof tape. At the foundation walls, the plastic goes up over the wall plastic and is fastened mechanically (see photo at top right) and then taped.
Step 4. Once all the plastic is fastened and sealed at the foundation wall, we insulate the walls. We attach foamboard or fiberglass batts mechanically so that it's there to stay. In this house we used 2" thick extruded polystyrene that yields an R-value of 10. Sometimes we use R-10 polyisocyanurate foam board, and other times we spray closed cell foam or attach fiberglass batts on the foundation walls. In all cases we choose the best product for the application. Notice also the two separate pink blocks where the crawl space vents are. Foam board fills the big hole, and then we use spray foam to seal the edges and make it airtight.
Step 5.To seal air leaks through the band joist as well as to insulate it, we spray foam insulation on it. Here you can see a close up of one of the vents in the crawl space wall with the foam board plug. Also, notice the strip of unsealed concrete at the top of the foundation wall and around the vents. This is necessary for termite inspections.
Step 6. Since the crawl space needs to sealed away from external environments, we also need to make sure the crawl space door is airtight and insulated. The photo shown here is from a different house since the one above had no access to the outside. Also, this case was a little unusual in that the best solution here was to build a second door inside the crawl space. The foam board insulation you see on both doors here is polyisocyanurate.
Conclusion. Once we get all these steps completed (and they may not always happen in exactly the order shown here), the crawl space has an entirely different feel to it. It's no longer the nasty, yucky place it was when we started. Any contractors or homeowners who have to go into the crawl space after encapsulation are happy to go in because the smells, the bugs, and the general unpleasantness are gone. Better still, the homeowners who live above the crawl space can breathe easier because their air is cleaner. They also have a more comfortable, efficient, and durable house. If you're still wondering if you should encapsulate, consider these benefits.